This invention relates to scale and corrosion inhibitors for use in the treatment of water. The present invention will be described herein with particular reference to the treatment of water used for cooling in the course of industrial or chemical processes and water used in oilfield applications, although it is not to be construed as being limited thereto.
Efficient cooling is a vital part of many industrial and chemical processes. This is best achieved by the circulation of water through a heat-exchanger (e.g. cooling coils or a jacket) which, for maximum efficiency, should ideally be kept completely free of corrosion or scale deposits that can otherwise impair heat transfer.
Cooling water systems can be of the xe2x80x9conce-throughxe2x80x9d or xe2x80x9cevaporative recirculatingxe2x80x9d type. The latter type is much preferred, because the high enthalpy of vaporisation of water makes more efficient use of each unit mass of water passing through the systemxe2x80x94when compared with simply using its thermal capacityxe2x80x94leading to reduced water usage and hence lower chemical treatment, water-supply and disposal costs.
The cost savings are maximised by operating at high recycle rates, often termed xe2x80x9ccycles of concentrationxe2x80x9d or xe2x80x9cconcentration factor/ratioxe2x80x9d, (hereinafter CR). This can have the additional benefit of reducing the corrosivity of the cooling water, because the expulsion of carbon dioxide from the system leads to higher pH. However, the formation of insoluble mineral deposits, xe2x80x9cscalexe2x80x9d, particularly calcium and basic magnesium carbonates, is promoted by the increased concentrations of Ca++ and Mg++ ions and the high pH. Also, calcium sulphate can be problematic at high concentration ratios. Furthermore, the thermodynamic and kinetic factors tend to favour scale deposition in the hottest parts of the system, ie, the heat exchanger, where efficient removal of heat is most critical.
For these reasons, while the industry recognises the benefits of operating at high xe2x80x98CRxe2x80x99 values, this has often been compromised by the practical limit imposed by the increasing concentration of dissolved salts, particularly those which are potentially scale-forming.
The application of compounds, xe2x80x9cScale Inhibitorsxe2x80x9d, which are capable of inhibiting crystal nucleation and/or retarding the growth of scale forming minerals when applied in sub-stoichiometric amounts, is one approach which has been used successfully for many years. However, the currentlyxe2x80x94used scale inhibitors have limits in terms of the maximum scaling potential (and hence xe2x80x98CRxe2x80x99) that may be supported.
Thus, there is a widely-recognised need in the water treatment industry for new scale inhibitors, capable of operating under severe scaling conditions, to reduce the cost and the environmental impact of scaling. The impetus for this is most apparent in the industriallyxe2x80x94developed countries, where lack of good quality water and increasingly restrictive environmental legislation have combined to force users to operate cooling systems under increasingly severe scaling conditions which are at the limit of existing technology.
Even where compounds are highly cost-effective scale inhibitors, further constraints are imposed by the need for them to be thermally and hydrolytically stable, safe for operators to use, environmentally acceptable, and compatible with high levels of scaling cations and also other chemicals that may be added to the system, e.g. corrosion inhibitors, and biocides, both non-oxidising and, notably, oxidising biocides.
The applicant has found that the criteria listed in the immediatelyxe2x80x94preceding paragraph may be met by the use of scale inhibitors which comprise both oxyalkylene units and phosphonate units.
The applicant has also found that such inhibitors can also be used to inhibit corrosion caused by (or exacerbated by) water.
Accordingly, the present invention provides a scale inhibitor or corrosion inhibitor for use in the treatment of water, said inhibitor comprising at least one oxyalkylene unit and at least one phosphonate unit.
Each oxyalkylene unit may conveniently be derived from a dihydric or polyhydric alcohol. Suitable examples of such alcohols include ethylene glycol, propylene glycol, oligomers and higher polymers of those glycols (such as tri-ethylene glycol, tetra-ethylene glycol, poly(oxyethylene) glycol and poly(oxypropylene) glycol) as well as mixtures or copolymers of any two or more of said alcohols.
Each phosphonate unit may conveniently be derived from a mono- or di-phosphonic acid. Suitable examples of such acids include vinyl phosphonic acid (VPA) and vinylidene-1,1-di-phosphonic acid (VDPA).
Alternatively, each phosphonate unit may be derived from a salt of a mono- or di-phosphonic acid.
In a particularly preferred embodiment, the present invention provides a scale or corrosion inhibitor having the general formula: 
In the formula, R1 and R2 (which may be the same or different) denote H, alkyl, aryl, or alkoxy and (n+m)=1 to 100, to form a homo-, random mixed or block co-polymer; 
Z2 is H, alkyl, aryl or Z1;
R3 and R4 are each independently alkyl, aryl, alkoxy, or H; Y is PO3X2, alkyl, aryl, alkoxy or H;
X is alkyl, H, alkali metal, or amine salt.
Thus, an inhibitor according to the present invention may consist essentially of the reaction product of an oligomeric oxyalkylene glycol (e.g. triethylene glycol or tetra-ethylene glycol) and a di-phosphonic acid (e.g. vinylidene-1,1-di-phosphonic acid).
Alternatively, inhibitors according to the present invention may consist essentially of the reaction product of an alkylene oxide (e.g. ethylene oxide or propylene oxide) with a hydroxy-phosphonate (or its salt or ester) having the general formula: 
wherein Z2 is H, alkyl, aryl or Z1;
R3 and R4 are each independently alkyl, aryl, alkoxy, or H;
Y is PO3X2, alkyl, aryl, alkoxy or H;
X is alkyl, H, alkali metal, or amine salt.
For example, the hydroxy phosphonate may comprise 1-hydroxyethane-1, 1-diphosphonic acid (HEDP). Inhibitors according to the present invention have been found to be particularly effective in the treatment (for scale inhibition) of water containing upwards of 600 ppm alkaline earth metal ions, expressed as ppm calcium carbonate.
The present invention also provides a method of treating water, said method comprising the addition thereto of an effective amount of an inhibitor according to the present invention, as hereinbefore described.
The water may be, for example, cooling water, oilfield water, water used in paper manufacture, water in a hydraulic system, water in a desalination system (including membranexe2x80x94process and evaporative process desalination systems), boiler water, geothermal water or water in an irrigation system.
Under optimum conditions of use, the inhibitors according to the present invention are at least partially biodegradable.